The invention relates to a low-pressure mercury vapor discharge lamp comprising a light-transmitting discharge vessel,
said discharge vessel enclosing, in a gastight manner, a discharge space provided with a filling of mercury and a rare gas,
at least a part of an inner wall of the discharge vessel being provided with a transparent layer.
The invention also relates to a compact fluorescent lamp.
In mercury vapor discharge lamps, mercury constitutes the primary component for the (efficient) generation of ultraviolet (UV) light. A luminescent layer comprising a luminescent material (for example, a fluorescent powder) may be present on an inner wall of the discharge vessel to convert UV to other wavelengths, for example, to UV-B and UV-A for tanning purposes (sun panel lamps) or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of low-pressure mercury vapor discharge lamps is usually tubular and circular in section and comprises both elongated and compact embodiments. Generally, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of relatively short straight parts having a relatively small diameter, which straight parts are connected together by means of bridge parts or arc-shaped parts. Compact fluorescent lamps are usually provided with an (integrated) lamp cap.
It is known that measures are taken in low-pressure mercury vapor discharge lamps to inhibit blackening of parts of the inner wall of the discharge vessel, which parts are in contact with a discharge which, during operation of the discharge lamp, is present in the discharge space. Such blackening, which is brought about by interaction between mercury and the glass from which the discharge vessel is made, is undesirable and does not only lead to a reduction of the maintenance but also to an unaesthetic appearance of the lamp, particularly because the blackening occurs irregularly, for example, in the form of dark stains or dots.
A low-pressure mercury vapor discharge lamp of the type described in the opening paragraph is known from U.S. Pat. No. 4,544,997. In the known discharge lamp, an oxide selected from the group formed by yttrium, scandium, lanthanum, gadolinium, ytterbium and lutetium is used as the transparent layer. The oxide is provided as a thin layer on the inner wall of the discharge vessel. The known transparent layers are colorless, hardly absorb UV radiation or visible light and satisfy the requirements with respect to light and radiation transmissivity. The use of the known transparent layers causes blackening and discoloring of the inner wall of the discharge vessel of the low-pressure mercury vapor discharge lamp to be reduced.
A drawback of the use of the known low-pressure mercury vapor discharge lamp is that the maintenance still is relatively poor due to said blackening. As a result, in addition, a relatively large amount of mercury is necessary for the known lamp in order to realize a sufficiently long service life. In the case of injudicious processing after the end of the service life, this is detrimental to the environment.
It is an object of the invention to provide a low-pressure mercury vapor discharge lamp of the type described in the opening paragraph, having an improved maintenance.
To this end, the low-pressure mercury vapor discharge lamp according to the invention is characterized in that
the discharge vessel is made from a glass comprising silicon dioxide and sodium oxide,
with the glass composition comprising the following essential constituents, given in percentages by weight (wt. %):
and in that the transparent layer comprises a borate and/or a phosphate of an alkaline earth metal and/or of scandium, yttrium or a further rare earth metal.
A discharge vessel of a low-pressure mercury vapor discharge lamp according to the invention having the above glass composition and comprising a transparent layer including said borate and/or phosphate appears to be very well resistant to the action of the mercury-rare gas atmosphere which, in operation, prevails in the discharge vessel of the low-pressure mercury vapor discharge lamp. As a result, blackening due to interaction between mercury and the glass from which the discharge vessel is manufactured is reduced, resulting in an improvement of the maintenance. During the service life of the discharge lamp, a smaller quantity of mercury is withdrawn from the discharge, so that, in addition, a reduction of the mercury consumption of the discharge lamp is obtained and in the manufacture of the low-pressure mercury vapor discharge lamp a smaller mercury dose will suffice.
Blackening caused by withdrawing mercury from the discharge occurs in straight parts as well as arc-shaped parts of the low-pressure mercury vapor discharge lamp. In general, blackening is reduced by providing the inner wall of the discharge vessel with a sufficiently adherent and sufficiently thick transparent layer. In general, the arc-shaped lamp parts of compact fluorescent lamps are more subject to blackening than the straight lamp parts. The arc-shaped lamp parts are generally not bent until after the tubular discharge vessel has been provided with the transparent layer and, if necessary, a luminescent layer. In the bending operation, the thickness of the transparent layer in the arc-shaped lamp parts is reduced and the transparent layer is stretched, which may result in the formation of cracks in the transparent layer. Crack formation occurs in particular in the known discharge lamp wherein an oxide selected from the group formed by yttrium, scandium, lanthanum, gadolinium, ytterbium and lutetium is used as the transparent layer. The application of a transparent layer according to the invention in combination with the sodium-rich glass in accordance with the invention causes blackening to be substantially reduced in the straight parts as well as the arc-shaped parts of the low-pressure mercury vapor discharge lamp.
The measure according to the invention is notably suitable for compact fluorescent lamps having arc-shaped lamp parts, wherein the discharge vessel is additionally surrounded by a light-transmitting envelope. The temperature of the discharge vessel of such xe2x80x9ccoveredxe2x80x9d compact fluorescent lamps is comparatively high because the heat dissipation to the environment is reduced by the presence of the envelope. This unfavorable temperature balance adversely affects the maintenance of the known discharge lamp due to an increased level of blackening. In experiments it has surprisingly been found that the maintenance of a compact fluorescent lamp provided with a low-pressure mercury vapor discharge lamp according to the invention, the discharge vessel of which is surrounded by an envelope, exceeds 90% after 2000 burning hours, while the maintenance of an identical compact fluorescent lamp provided with the known low-pressure mercury vapor discharge lamp, the discharge vessel of which is surrounded by an envelope, is less than 80% after 2000 burning hours.
An additional advantage of the use of a discharge vessel in accordance with the invention in low-pressure mercury vapor discharge lamps is that the glass is comparatively inexpensive. In the known discharge lamp use is made of a so-called mixed alkali glass having a comparatively small SiO2 content and comprising, inter alia, approximately 8 wt. % Na2O and 5 wt. % K2O. The cost price of said glass is comparatively high.
A comparison between the composition of the known glass and the glass in accordance with the invention shows that the alkali content is different. The glass in accordance with the invention is a so-called sodium-rich glass with a comparatively low potassium content, while the known glass is a so-called mixed alkali glass having an approximately equal molar ratio of Na2O and K2O. An advantage resides in that the mobility of the alkali ions in the sodium-rich glass is comparatively high with respect to the mobility in the mixed alkali glass. In addition, melting of sodium-rich glass is comparatively easier than melting mixed alkali glass.
The transparent layer in the low-pressure mercury-vapor discharge lamp in accordance with the invention further satisfies the requirements with respect to light and radiation transmissivity and can be easily provided as a very thin, closed and homogeneous transparent layer on an inner wall of a discharge vessel of a low-pressure mercury vapor discharge lamp. This is effected, for example, by rinsing the discharge vessel with a solution of a mixture of suitable metal-organic compounds (for example, acetonates or acetates, for example, scandium acetate, yttrium acetate, lanthanum acetate or gadolinium acetate mixed with calcium acetate, strontium acetate or barium acetate) and boric acid or phosphoric acid diluted in water, while the desired transparent layer is obtained after drying and sintering.
An additional advantage of the use of a transparent layer according to the invention in low-pressure mercury vapor discharge lamps is that such layers have a relatively high reflectivity in the wavelength range around 254 nm (in the discharge vessel, mercury generates, inter alia, resonance radiation having a wavelength of 254 nm). Given the refractive index of the transparent layer, which is relatively high with respect to the refractive index of the inner wall of the discharge vessel, such a layer thickness is preferably chosen that the reflectivity at said wavelength is maximal. By using such transparent layers, the initial light output of low-pressure mercury vapor discharge lamps is increased.
The glass composition preferably includes the following constituents:
The composition of such a sodium-rich glass is similar to that of ordinary window glass and it is comparatively cheap with respect to the glass used in the known discharge lamp. The cost price of the raw materials for the sodium-rich glass as used in the discharge lamp in accordance with the invention is only approximately 75% of the cost price of the raw materials for the mixed alkali glass as used in the known discharge lamp. Moreover, the conductance of said sodium-rich glass is comparatively low; at 250xc2x0 C. the conductance is approximately.
log xcfx81=6.3.
while the corresponding value of the mixed alkali glass is approximately
log xcfx81=8.9.
In a preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention, the transparent layer comprises a borate and/or a phosphate of calcium, strontium and/or barium. Such a transparent layer has a relatively high coefficient of transmission for visible light. Moreover, low-pressure mercury vapor discharge lamps with a transparent layer comprising calcium borate, strontium borate or barium borate or calcium phosphate, strontium phosphate or barium phosphate have a good maintenance.
In a particularly preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention, the transparent layer comprises yttrium borate and strontium borate. Such a transparent layer has a relatively high coefficient of transmission for ultraviolet radiation and visible light. It has further been found that a transparent layer comprising yttrium borate and strontium borate is only slightly hygroscopic and adheres well to the inner wall of the discharge vessel. Moreover, the layer can be provided in a relatively simple manner (for example, with yttrium acetate and strontium acetate mixed with boric acid), which has a cost-saving effect, notably in a mass manufacturing process for low-pressure mercury vapor discharge lamps.
In practical embodiments of the low-pressure mercury vapor discharge lamp, said transparent layer has a thickness of approximately 5 nm to approximately 200 nm. At a layer thickness of more than 200 nm, excessive absorption of the radiation generated in the discharge space takes place. At a layer thickness of less than 5 nm, there is interaction between the discharge and the wall of the discharge vessel. Such layer thicknesses are customarily obtained by means of optical measurements. A layer thickness in the range from approximately 50 nm to approximately 90 nm is particularly suitable. In said preferred range, the transparent layer has a relatively high reflectivity in the wavelength range around 254 nm.
A further preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that a side of the transparent layer facing the discharge space is provided with a layer of a luminescent material. An advantage of the use of a transparent layer according to the invention in low-pressure mercury vapor discharge lamps is that the luminescent layer comprising a luminescent material (for example, a fluorescent powder) adheres significantly better to such a transparent layer than to a transparent layer of the known low-pressure mercury vapor discharge lamp. Said improved adhesion is obtained particularly in the arc-shaped parts of low-pressure mercury-vapor discharge lamps.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.